In a LCD, various voltage signals are applied to LCD elements to change states of liquid crystal so as to change transmittance and gray or color levels. Take a 256-level display as an example, the 256 levels are indicated by 8 bits, and as shown in the plot of FIG. 1, voltage values in the vertical axis respectively corresponding to gray/color levels 0˜255 in the horizontal axis are selectively applied to the LCD pixels.
Generally, data are updated every frame in a LCD. Viewing from a single LCD pixel, an applied voltage readily varies with a given level data. However, the response speed of liquid crystal is not definitely quick as well. Response speed is typically defined by a period of time required for achieving 10%˜90% of expected luminance from the current luminance.
Generally, response speed significantly decreases in a low-temperature environment. A machine like a vehicular navigation system used in for example Northern Europe even possibly needs to be started in a temperature as low as minus tens of degrees Centigrade. In such a low temperature, liquid crystal is too viscous to be well responsive while starting. Therefore, the resulting image is vague and poor displaying quality is rendered.
A method having been developed for enhancing response speed of liquid crystal is known as “overdrive”.
An overdrive method is a technique applying a voltage higher than a voltage determined according to a given data level, e.g. 0˜255, to accelerate the change of the LC state. The higher voltage, for example, is a voltage corresponding to a level higher than the given data level.
For precisely controlling overdrive voltages depending on images, another conventional overdrive method is proposed to predict level data for each pixel in the previous frame and then output overdriven level data accordingly, as disclosed in Japanese Patent Publication No. 2005-107531.
Since the overdrive operation in Japanese Patent Publication No. 2005-107531 is updated every frame, and it is known the level change between adjacent frames could be insignificant, the predicted values are likely to have no or almost no change. Then the overdrive effect cannot be seen.
Therefore, the inventor of the present application proposed in a previously filed Japanese patent application No. 2008-111730 a method for enhancing the response speed of a LCD device by utilizing temperature-dependent lookup tables containing predicted levels after a predetermined number of frames. For example, even if molecules are relatively inactive and response speed is relatively low in a low temperature environment, overdrive operations of the same number as the predetermined number of frames can still be performed according to preset overdrive values and the predicted levels after the predetermined number of frames.
However, in the architecture of the previously filed Japanese patent application, control is performed, usually for all the pixels, by calculating drive voltages while referring to predicted values. Furthermore, since all the combinations of levels at the start and the end of the overdrive are required to be kept in the lookup tables for storing overdrive values, the requirement of a relatively large memory capacity would be a problem.
For example, for preparing lookup tables corresponding to 14 levels of temperature from −30° C. to 35° C., including −30, −25, −20, −15, −10, −5, 5, 0, 10, 15, 20, 25, 30 and 35° C., in order to display in a 8-bit grey scale, a memory capacity of 256×256×8×14=7.3 Mbit is required. Once conditions such as display gray scales, temperatures, etc. are set in more levels, even great memory capacity becomes necessary. Cost would be raised accordingly.
In one way, on a condition that the response time of liquid crystal is highly sensitive to temperature, the interval 5° C. of the lookup tables may be too large in some cases. Therefore, once temperature changes dramatically, the switching between different lookup tables would result in a problem of obvious difference in image quality.
Consequently, although it is preferred to perform precise control in response to detailed temperature setting, the limitation in the memory capacity makes it difficult to practice.
Furthermore, for a device such as particularly a LCD TV, due to high-speed motion pictures, the response speed is required to be high. Therefore, driving operations are performed at a speed of two times, four times, etc. However, as the device is generally produced at a room temperature such as 25° C., it is hard to follow by high-speed drive under a temperature drop up to for example 10 degrees. Consequently, it is to be considered whether the feedback control using the above-mentioned lookup tables is proper or not.
With regards to the memory capacity, the above-mentioned Japanese patent application No. 2005-107531 discloses the use of a pair of one-dimensional tables to reduce the memory capacity for storing overdrive values. However, the preset overdrive values cannot exhibit the best performance for display functions of the LCD device.